The invention relates to an engine braking method for a supercharged internal combustion engine.
An engine braking method is known from DE 196 37 999 A1. The internal combustion engine is equipped with an exhaust gas turbocharger, the exhaust gas turbine of which is provided with a variable turbine geometry for variably setting the effective turbine inlet flow cross section. To achieve high engine braking power, the variable turbine geometry is adjusted into a blocking position reducing the cross section, with the result that the exhaust back pressure between the cylinder outlets and the exhaust gas turbine rises. The engine braking power is regulated via a controller in the internal combustion engine as a function of current engine and operating variables and also of the driver's instruction by a variation in the opening curve of a throttle valve on the cylinders. This throttle valve opens toward the end of the compression stroke and maintains its opening position until the end of the expansion stroke, whereas, during the inlet stroke, the throttle valve remains closed. The engine braking method is in the form of a four-stroke method in which the pistons in the cylinders have to perform expulsion work counter to the exhaust gas backpressure.
U.S. Pat. No. 6,293,248 B1 discloses a two-stroke engine braking method for a supercharged internal combustion engine. In this braking method, combustion air which is under boost pressure is supplied to the cylinders by the control of charge exchange valves, that is to say of inlet and outlet valves, is compressed there and is subsequently discharged into the exhaust tract. This takes place in that, shortly before top dead center is reached, the outlet valve is opened and, after top dead center is overshot, is closed again. Before the outlet valve is closed, in the expansion stroke, the inlet valve is opened, so that, during an overlap phase, both the inlet valve and the outlet valve are open. The inlet valve remains open over a relatively long crank angle range, specifically over and beyond bottom dead center into a middle piston lift range of the compression stroke. Only approximately in the middle between bottom and top dead center is the inlet valve closed during the compression stroke, so that, also only thereafter, all the valves are closed and the maximum pressure build-up can be achieved in the cylinder. However, because of the late inlet closing time point and the already shortly following outlet opening time, the angular range in which all the valves are closed during the compression stroke is restricted to a relatively small crank angle range. Correspondingly, only a low maximum pressure built up can be achieved.
It is the object of the present invention to provide an engine braking method for a supercharged internal combustion engine, by means of which a high braking powers, along with a relatively low mechanical and thermal loads, can be implemented.